1. Technical Field
The present invention relates to a discharge lamp, a manufacturing method for the discharge lamp, a light source device, and a projector.
2. Related Art
As a light source of a projector, a discharge lamp such as an ultra-high pressure mercury lamp is known. In the discharge lamp of this type, a pair of electrodes made of metal such as tungsten is disposed to be opposed to each other in the internal space of a luminous tube made of quartz glass and is encapsulated together with mercury and a halogen gas. As a manufacturing method for the electrodes, there is known a method of winding a lead wire of metal on a core material in a coil shape and thereafter melting a part of the lead wire and forming a distal end portion functioning as a start point of electric discharge.
For example, Japanese Patent No. 4400095 (Patent Literature 1) discloses a short-arc type ultra-high pressure mercury lamp including electrodes including projecting sections, large diameter sections, coil sections, and shaft sections. In the ultra-high pressure mercury lamp, coils have a double wound structure in which the coils are wound on the shaft sections from the distal ends toward the bases of the electrodes and thereafter folded back in the bases and wound from the bases toward the distal ends. The end portions of the coils located on the outer surfaces are melted and integrated with the large diameter sections.
Incidentally, examples of a factor of reducing the life of the discharge lamp include devitrification of the luminous tube and an increase in an inter-electrode distance due to wear of the electrodes. It is known that a factor of the devitrification is electric discharge that occurs between the electrodes and the luminous tube, in particular, electric discharge that occurs between the rear end portions of the electrodes, which have a short distance to a luminous tube wall, and the inner wall of the luminous tube. As a method of suppressing the increase in the inter-electrode distance, a driving method for the discharge lamp is often adopted. According to the driving method for the discharge lamp, a waveform of a driving current supplied to the discharge lamp is contrived to facilitate formation of protrusions at electrode distal ends to thereby maintain an appropriate inter-electrode distance.
However, even if the inter-electrode distance is maintained by the driving method for the discharge lamp, unless the electrodes have a size (a capacity) enough for withstanding the driving method, the electrodes are worn by heat, for example, when the driving current supplied to the discharge lamp is large. Therefore, the electrode capacity needs to be set large to a certain degree. For example, as in the electrode forming method of Patent Literature 1, when the coils are doubly wound on the shaft sections, the coils are wound on the shaft sections long in order to increase the weight of the electrodes. In such a method, the entire shape of the electrodes is long in an extending direction of the shaft section. An electrode substance supplied to form protrusion sections in the large diameter sections at the electrode distal ends is present in a position far from the large diameter sections at the electrode distal ends where arc discharge occurs. Therefore, it is difficult to maintain the protrusion sections. The inter-electrode distance increases. Even if the coil end portions are melted, since the coils having a diameter enough for sufficiently securing the electrode capacity through the double winding are used, it is difficult to secure a long distance between the electrode rear end portions and the inner wall of the luminous tube. It is difficult to suppress electric discharge between the electrodes and the luminous tube.